The main components of a drive train of a motor vehicle are a drive aggregate and a transmission. The transmission changes rotational speed and torque, thus providing a traction force by the drive aggregate to the output of the drive train. The present invention relates to a method for operating a drive train were the drive aggregate comprises at least an electric machine. The inventive method is therefore applicable in a pure electric vehicle, in which the drive aggregate is exclusively an electric machine, as well as for a hybrid vehicle in which the drive aggregate is an electric machine and a combustion engine. The electric machine of the respective drive aggregate is hereby, for instance, designed as a three-phase current synchronous machine.
In a three-phase current synchronous machine, the rotational speed of a rotor corresponds to the rotational speed of a rotating electromagnetic field, thus concluding that the rotor of such a synchronous machine rotates synchronously to the rotating field. A three-phase current synchronous machine, were the rotating field is generated by means of a frequency converter, has therefore no slippage. At that time, when the rotor of the three-phase current synchronous machine is at stand still, mainly just one phase of the synchronous machine has a current flow through it, thus causing a one-sided stress of the related components of the power electronic in the frequency converter. The frequency converter of a three-phase current synchronous machine comprises mainly a rectifier, which represents an interface to a DC intermediate circuit, or DC voltage intermediate circuit, respectively, as well as of an inverter which is fed from this DC intermediate circuit or DC voltage intermediate circuit, respectively.
The inverter comprises switching transistors and generates a pulse width modulated voltage. The inductance of the electric machine creates a smoothing of the current, whereby the amplitude of the resulting output voltage and its frequency can be regulated.
At the time, when a three-phase current synchronous machine is at standstill, its maximal available torque is, for a defined time interval, less than with a rotating synchronous machine, because the components of the power electronics in the frequency converter in a rotating synchronous machine are alternately charged. However, at its stand still, the power electronics are strained one-sided which can cause overheating and thus damage to the power electronics of the frequency converter of the synchronous machine. This causes difficulties during the start of a motor vehicle where the drive aggregate is an electric machine which comprises a three-phase current synchronous machine. Therefore, there exists a need for a method to operate a drive train of a motor vehicle, through which on one hand reliable starting can be realized, and through which on the other hand an overload and thus damage of the electric machine, designed as a three-phase current synchronous machine, can be avoided.